With the emergence of a more aged society in recent years, the bone disease termed osteoporosis has been becoming a problem. In this disease the calcium is withdrawn from the bones leaving them friable and prone to fracture at the slightest impact; and it is one motive for concern in old people. Physical diagnosis is performed mainly by determining the density of bone precisely by means of diagnostic apparatus employing X-rays, typified by DXA and DCT; however, physical diagnosis by means of X-rays is beset by various problems such as the fact that the apparatus is large, and that its use is restricted by the need to prevent harmful radiation exposure.
Accordingly, diagnostic apparatus employing ultrasound has started to become popular because the equipment is simple and does not cause such problems. Diagnostic apparatus employing ultrasound measures the speed and attenuation of ultrasound waves propagated inside bony tissues, and uses this to estimate the density and elastic modulus (elastic strength) of the bone. If a low estimated value is obtained it can be deduced that this is because of withdrawal of calcium from the bone, and hence osteoporosis is diagnosed.
For example, in the diagnostic apparatus recorded in Japanese Laid-Open Patent Application No. H2-104337 (U.S patent application No. 193,295) the speed of sound in bony tissue is measured by placing two ultrasonic transducers facing each other on either side of the bony tissue of an examinee chosen as the measurement site, emitting ultrasonic pulses towards the bony tissue from an ultrasound transducer on one side and receiving the ultrasonic pulses transmitted by the bone tissue at the ultrasound transducer on the other side, in order to determine the speed of sound within the bone; and progress in osteoporosis is diagnosed when the speed of sound inside the bony tissue is slow. This is because the data processing algorithm of this diagnostic apparatus is based on the working premise that the speed of sound in bony tissue is proportional to bone density.
However, there is no firmly established theoretical basis for linking bone density and the speed of sound: strictly speaking the speed of sound in bony tissue is not proportional to bone density, but is given by the square root of "the elastic modulus of the bone/bone density". And since the elastic modulus of bone rises as bone density increases, and therefore the modulus of elasticity of bone and bone density contribute to the speed of sound in such a way that they cancel one another out, the speed of sound in bony tissue cannot respond sensitively to an increase in bone density, and the coefficient of correlation between the speed of sound in bony tissue on the one hand and bone density on the other is not at all high. There is also no established theoretical basis for linking bone density and attenuation of ultrasound waves.
Therefore, it is unreasonable to expect highly reliable diagnoses from prior diagnostic apparatus which estimate bone density and the elastic modulus of bone on the basis if results of determination of attenuation of ultrasound waves, or determination of the speed of sound in bony tissue.
As means of overcoming these inadequacies, Japanese Patent Applications No. H6-310445, No. H7-140730 and No. H7-140734, applied for by the present applicant, propose ultrasonic pulse-echo type apparatus for diagnosing osteoporosis in which a single ultrasonic transducer is used; ultrasonic pulses are emitted repeatedly towards smooth-surfaced bony tissue; the echoes returned from the bony tissue are received; the maximum echo (which can be regarded to be the echo due to perpendicular reflection) is extracted from the echoes received; the reflection coefficient and acoustic impedance, etc., of the bone are calculated from the maximum echo extracted, and the calculated values are used as the basis for diagnosing osteoporosis.
However, although it is possible, with the ultrasonic pulse-echo type apparatus for diagnosing osteoporosis claimed in the applications mentioned above, to emit adequate planewave ultrasonic pulses towards smooth-surfaced bony tissue and to receive back perpendicularly reflected echoes when the transducer is in contact with the skin of the examinee, there is the problem that the procedure is complex, because the direction of the transducer has to be altered in order to locate the maximum echo, and this requires skill and effort on the part of the person carrying out the procedure.
This invention is a response to the situation above, and its object is to offer an apparatus for diagnosing osteoporosis and a method for diagnosing osteoporosis which can give highly reliable diagnoses, by using a simple procedure which does not involve exposure to radiation.